[clang] Behavior of `C_INCLUDE_DIRS` on Linux differs from glibc to musl

The Linux clang driver tries to add the resource-include-dir ahead when using glibc, while it is delayed on musl.

On glibc, the resource-include-dir is added before detecting C_INCLUDE_DIRS. On musl, the resource-include-dir will not be included, since the function immediately returns after `C_INCLUDE_DIRS`.

The issue points to the Linux clang driver logic handling C_INCLUDE_DIRS and resource-include-dir, but names no files or tests. Start by locating that driver code and comparing the glibc and musl branches; confirm the intended ordering and inclusion behavior for both libc paths, then verify the result on each.

Written by the indexing model from the issue text.